I used First Book AI just as a test, honestly, as I've actually just finished my first draft of my actual book. That one required analysis and setting of themes from actual transcripts of interviews so with that I had to use multiple AIs actually. The final one was predominantly done with Perplexity but I wanted to test out First Book AI just to see what sort of draft you could put together on the same topic without all those detailed transcriptions. Honestly it was pretty good.

Overall I think it's a great tool to use for your first draft and then to do your own editing and adding your own personality from that really good product, a lot better than the other AI book writing software that I've tried in the past, which was fairly useless actually.